The United States has concluded that the Syrian government used chemical weapons in its fight against opposition forces, and President Obama has authorized direct U.S. military support — small arms and ammunition — to the rebels, the White House said Thursday.

"The president has said that the use of chemical weapons would change his calculus, and it has," said Benjamin Rhodes, Obama's deputy national security adviser. Rhodes said U.S. intelligence had determined with "high certainty" that Syrian government forces have "used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year."

Intelligence agencies estimate that 100 to 150 people have died as a result of chemical weapons use, he said.

Rhodes did not detail what he called the expanded military support, but it is expected initially to consist of light arms and ammunition. He said the shipments would be "responsive to the needs" expressed by the rebel command.

Obama has "not made any decision" to pursue a military option such as a no-fly zone and has ruled out the deployment of U.S. ground troops, Rhodes said.

Syria's outgunned rebels issued urgent appeals this week for antitank and antiaircraft weaponry to counter a government offensive that is backed by Hezbollah fighters and Iranian militia forces.

"Suffice it to say this is going to be different in both scope and scale," Rhodes said of the new assistance. Obama said last year that confirmation of chemical weapons use would cross a "red line" for the United States.

The shipments, to begin in a matter of weeks, are likely to be undertaken by the CIA, which has been the primary U.S. government interlocutor with the opposition's Supreme Military Council, led by Salim Idriss. Such covert action requires a signed presidential finding.

That method avoids what the administration previously has said are legal restraints against supplying arms for attacks against another government without approval by an international body such as the United Nations, the Washington Post reported, citing U.S. officials who spoke on the condition of anonymity.

The weapons would probably be delivered by air to Turkey or Jordan, or both, and by land into Syria along rebel-held corridors. The opposition's requests for antitank and antiaircraft weaponry are still under discussion.

Despite its long insistence that Syrian President Bashar Assad must leave power and its recognition of the opposition as "a legitimate representative of the Syrian people," the United States and most of the world still officially recognize the Assad government.

Rhodes said the administration would continue to push for a negotiated political settlement of the conflict, including a proposed conference between opposition leaders and government representatives that is on hold.

Syria will be at the top of the agenda when Obama meets with leaders of the Group of Eight industrial nations, including Russia, in Northern Ireland next week. Russia, Assad's primary arms supplier and diplomatic backer, has blocked harsher international action against him at the United Nations.

The Obama administration has provided more than $515 million in humanitarian and nonlethal military assistance to the Syrian opposition, including food and medicine. This week, the United Nations put the death toll in the conflict, which is in its third year, at more than 90,000. Millions have been displaced inside the country, and more than 1.5 million Syrian refugees are in neighboring countries.

But while regional governments have been funding weapons supplies for the rebels, the United States and its closest allies, including Britain and France, have been reluctant to do so.

This week, demands from numerous U.S. lawmakers that Obama authorize the delivery of arms — despite persistent U.S. public reluctance reflected in opinion polls — escalated after the rebels' loss of a key town, Qusair, near the Lebanese border and amid reports that government forces were massing with Hezbollah and Iranian fighters to retake rebel-held portions of the northern city of Aleppo.

Officials described Obama's decision as gradual, as intelligence assessments about chemical weapons use became more firm. After an initial, inconclusive assessment in April, the president "directed our intelligence community to further investigate the use of chemical weapons and to seek credible and corroborative information," Rhodes said in a briefing for reporters Thursday afternoon.

The U.S. investigation, he said, was conducted separately from, but in conjunction with, a U.N. effort to confirm chemical weapons use.

Intelligence committees in Congress were briefed on the arms decision earlier this week and on the chemical weapons conclusions on Thursday.

Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina welcomed the chemical weapons assessment. The two have been among the sharpest critics of the administration, saying it has not been doing enough to help the rebels.

"U.S. credibility is on the line," they said in a joint statement. "Now is not the time to merely take the next incremental step. Now is the time for more decisive actions," they said, such as using long-range missiles to degrade Assad's air power and missile capabilities.

Sen. Robert Casey Jr., D-Pa., said the moderate opposition forces risk defeat without heavier weapons, but he also warned that may not be enough.

"The U.S. should move swiftly to shift the balance on the ground in Syria by considering grounding the Syrian air force with stand-off weapons and protecting a safe zone in northern Syria with Patriot missiles in Turkey," Casey said.

Such a no-fly zone would roughly parallel the NATO action in Libya two years ago. NATO has repeatedly said that it is not contemplating a similar move in Syria.

The chemical weapons assessment closes an awkward chapter for the Obama administration, in which it lagged behind two key European allies in reaching the same conclusion. France sent its dossier of chemical weapons evidence to Washington more than a week ago, and Britain had earlier provided what both countries called extremely persuasive evidence.

"A line has been crossed. Unquestionably," French Foreign Minister Laurent Fabius said last week.

"We've prepared for many contingencies in Syria," Rhodes said. "We are going to make decisions on further actions on our own timeline."

The announcement Thursday followed days of intensive discussions among Obama's top national security aides that administration officials said were partly a response to the rebel defeat in Qusair last week.

Idriss had spent days lobbying the White House and Congress for more help, arguing that the rout in the border town undermined his military authority with Syrian civilians and opened the way for a government onslaught in Aleppo.

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